Brazing is a widely-used method for joining metallic parts. Brazing generally involves applying a braze filler material (e.g., a braze paste or a braze foil) at a joint gap between two closely fitting parts, followed by heating of the braze filler to a temperature sufficient to melt the braze filler to a braze liquid. The braze liquid may then distribute across the joint gap and wet the surfaces of the parts being joined by capillary action. In general, the braze filler may be applied at one side of the joint gap and may be permitted to flow through the joint gap to the other side when melted. When suitably distributed across the joint gap, the braze liquid may cool and harden to provide a brazed joint between the two parts.
In cases where excess braze filler is applied to the joint gap, the excess braze liquid may drip from the joint gap and flow to other locations of the part. If the flow of the excess braze liquid is uncontrolled, the excess braze liquid may drip randomly over the part, possibly reaching undesirable locations such as mating surfaces, fluid holes, or high stress regions of the part. For example, if the excess braze liquid flows over mating surfaces, the ability of the part to assemble with a mating part may be compromised. As another example, cooling or other fluid holes may be blocked by excess braze liquid, thereby interfering with the function of the part. In addition, loss of excess braze material may pose economic disadvantages as well, such as when precious metals are used in the braze filler composition.
Careful control of the braze filler material volume (or mass) and/or the joint gap may be used to prevent the dripping of excess braze liquid over the part. However, depending on the part geometry, it may be difficult or cumbersome to precisely control the applied volume of braze filler and/or the joint gap, particularly when joining large parts and/or parts having a lot of surface variation.
Another known technique for controlling the flow of excess braze liquid involves the use of braze stop coatings that are brushed or sprayed on selected surfaces to protect them from excess braze flow. For example, U.S. Pat. No. 4,439,250 teaches the use of a braze stop coating that is impervious braze flow. Although effective, braze stop coatings may be limited by the tendency of the excess braze liquid to flow around the protected areas. In addition, it may be a challenge to apply braze stop coatings without interfering with the integrity of the braze joint. Furthermore, the use of braze stop coatings may be labor intensive in some cases, as it may involve both the application of the braze stop coatings as well as removal of the coatings.
Clearly, there is a need for improved brazing strategies for controlling excess braze flow.